This invention relates to the field of infrared sensitive imaging apparatus and more particularly to infrared linescan sensors for airborne reconnaissance.
Infrared linescan sensors have established themselves as one of the preferred sensors for airborne reconnaissance. They are capable of obtaining valuable tactical reconnaissance imagery during both day and night and under atmospheric conditions which preclude the use of conventional photographic or television-type electro-optical sensors. The value of this tactical imagery has been recognized by potential enemies so that they have equipped their forces with very extensive and effective anti-aircraft capability. Manned and unmanned reconnaissance aircraft have been forced to fly at extremely low altitudes with high velocity in order to increase their probability of survival. The velocity-to-height ratio (V/H) thus becomes a dominant factor in specifying and designing any sensor for such a low altitude, high V/H imagery collection mission.
In a linescan infrared sensor this is particularly true, because in such sensors, the transverse scan is accomplished by the rotation of a facetted scan mirror which has from 2 to 4 facets in typical designs known to the art. The transverse scan motion is repetitive as the mirror rotates at constant RPM. The scan motion in the other direction is accomplished by the forward motion of the aircraft which is usually taken to be constant. At very low values of V/H, it is sufficient to scan with a single infrared detector in order to cover the ground below the aircraft in a complete, or contiguously sampled manner. As the V/H increases to progressively higher values, the sampling rate must increase so as to avoid having gaps in the alongtrack, or flight, direction. By adding more detectors in a linear array parallel to the flight track, it is possible, in theory, to configure a sensor which could meet any required V/H expected in the near future using our present aircraft technology.
In actual practice there is a limit to the number of infrared detectors which can be used. There are a number of reasons why this is so. One of the most important being that of high cost. The infrared detectors used in such systems are themselves very expensive. In addition, each detector requires a preamplifier and, in the typical case of a photoconductive detector, individual detector bias circuits are required as well. The signal chain does not usually stop at the preamplifier. There are postamplifiers with automatic gain control, level clamping and automatic level adjust circuits. It also is necessary to equalize all channels to provide an equal output signal for an equal input radiance. Each of the above-described circuits add cost and complexity to the sensor. Given the above factors, there is a clear need to minimize the number of detector channels in parallel.
In most tactical airborne reconnaissance sensors, one of the chief problems has been that such sensors generate enormous data rates. The instantaneous data rate of the one such known system, for example, is 15.2 MHz. If this analog data were to be digitized into eight bit words, for example, the bit rate would be 30.4.times.8.times.10=243.2 megabits/sec. Some reduction in bit rate is possible using data stretching to fill the dead time of the scan line to line period, but even when this is done, the resulting data rate is excessively large for image manipulation, data linking, tape recording, or real-time display on a cathode ray tube. The invention provides a data rate reduction which matches the resolution in the end product imagery to the requirements of the human interpreter and to the limitations of the real image produced at the sensor focal plane.